Refine tensorflow doc (a follow up of pr #1253 to meet code review) (#1272)
Refine tensorflow doc (a follow up of pr #1253 to meet code review) (#1272)

* meet code review

* fix example

* meet code review

* refine title

* make saveTF compatible with saveCaffe

yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL